The use of liquid natural gas (LNG) as an alternative energy source for powering vehicles and the like is becoming more and more common as it is domestically available, environmentally safe and plentiful (as compared to oil). A use device, such as an LNG-powered vehicle, typically needs to store LNG in a saturated state in an on-board fuel tank with a pressure head that is adequate for the vehicle engine demands.
LNG is typically dispensed from a bulk storage tank to a vehicle tank by a pressurized transfer. While dispensing systems that saturate the LNG in the bulk tank prior to dispensing are known, they suffer from the disadvantage that continuous dispensing of saturated LNG is not possible. More specifically, dispensing of saturated LNG is not possible during refilling of the bulk tank or during conditioning of newly added LNG.
Another approach for saturating the LNG prior to delivery to a vehicle tank is to warm the LNG as it is transferred to the vehicle tank. Such an approach is known as “saturation on the fly” in the art. Examples of such “saturation on the fly” systems are presented in U.S. Pat. No. 5,687,776 to Forgash et al. and U.S. Pat. No. 5,771,946 to Kooy et al., the contents of which are hereby incorporated by reference.
Both the '776 and '946 patents disclose a bulk tank and a pump that pumps LNG from the bulk tank to a heat exchanger. A bypass conduit is positioned in parallel with the heat exchanger. A mixing valve permits a portion of the LNG stream from the pump to bypass the heat exchanger for mixture with the warmed natural gas exiting the heat exchanger in desired proportions to obtain the desired dispensing temperature for the LNG. The '776 and '946 patents both also disclose positioning an intermediate dispensing tank in circuit between the mixing valve and the dispensing line to the vehicle fuel tank. This permits pressure in the vehicle fuel tank to be relieved as the high pressure fluid from the vehicle fuel tank is returned to the intermediate dispensing tank in order to avoid mixing warm fluid with the cold LNG in the bulk tank.
While the vacuum jacketed intermediate dispensing vessel of the '776 and '946 patents is useful in storing heat from the piping and avoid it going back to the main storage tank, the system is not optimal. More specifically, moving the heat exchanger after an intermediate tank ensures the instantaneous flow of heated mass to the mixing valve while reducing the net volume of gas in the system. Gas is compressible and liquid is very nearly not compressible. As such, large gas volumes in the liquid flow from the pump to the vehicle tank compromise the net flow rate to the vehicle tank creating poor spray action in the tank and the possibility of short fills. A dispensing tank after the heat exchanger, as shown in the '776 and '946 patents, may well be eventually filled with liquid, but for some period of time during use it will have gas in it. While the gas flow through the mixing valve may allow for proper control, the empty vessel creates a problem in the hydraulics of the deliver to the vehicle tank.
Furthermore, saturation on the fly systems can generate a significant amount of unnecessary heat back to the main storage tank. This in turn can result in venting of natural gas, which is undesirable. Liquid left in piping that is of higher saturation than the storage tank will flash and send its heat back to the storage tank. Isolating the piping that is hot helps, but the trapped heat must be properly stored.
A need exists for a system and method for dispensing cryogenic liquids that addresses the above issues.